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1 Extremely slow write speeds after SSD upgrade
I have replaced built-in SSD and HDD with:
- Samsung 970 Evo Plus 2TB (got the latest firmware version)
- Samsung 860 Evo 2TB
At the first boot, in the recovery menu (alt+cmd+r) I have formatted both of these drive to APFS and merged them into one partition (diskutil resetfusion command). After installing Big Sur everything was fine - benchmarks reported ~3000MB/s write/read speeds, OS reported 4TB of free space in a single partition.
After a while (and filling my disks with 3.5TB of data) my write speeds are ridiculously slow - Blackmagic Disk Speed Test shows something between 90-300MB/s, and from time to time it even drops to zero and system freezes. Also, the read speeds are extremely inconsistent - one time it is 900MB/s, the other 1800MB/s (but usually around 1800MB/s).
I am wondering if this might be an overheat problem - while doing benchmarks I am getting:
- Temperature Sensor 1: 58 Celsius
- Temperature Sensor 2: 78 Celsius
And Samsung SSD Temps: 950, 960, 970, 750, 840, 850 and 860 EVO and PRO series are rated for operation between 0ºC and 70ºC.
In idle, when I am not doing anything the temps are not much better:
- Temperature Sensor 1: 51 Celsius
- Temperature Sensor 2: 66 Celsius
I also replaced CPU to i7-7700, the temps in idle are fine:
Fan: 1207 rpm
- CPU die temperature: 48.55 C
- GPU die temperature: 38.00 C
The first round of benchmarks gives better results (500MB/s / 2800MB/s), but everything after is limited - clearly a throttling does occur.
Have you encountered such issue? I can install some thermal pads and a radiator on M.2 SSD, but still - this does not seem normal. Can a cheap-ass adapter from Aliexpress be the reason?
Edit: In MacRumors: internal SSD is overheating, how to fix thread I found the following information:
P.S.: Also never fill up your SSD to more than 85% to 90% of its capacity. Mac OS creates lots of temporary files (logs, swap, etc.). Once only 10% to 15% of available disk space remain, the amount of disk I/O seems to pick up noticeable.
And someone suggested to do
sudo trimforce enable
Which I will try
Update (02/09/2021)
I’m not sure I follow why you created a Fusion Drive using these two SSD! Samsung 970 EVO Plus 2TB M.2 PCIe/NVMe SSD & Samsung 860 EVO 2TB 2.5” SATA SSD
2 reasons:
- I prefer to have one partition over many
- When I will be selling this Mac it is better to have it this way - an average Joe is too dump to grasp a concept of partitions
Basically, the PCIe SSD is not seen as a discreet drive within the macOS (look at your disk information pane under The About This Mac Storage tab all you’ll see is the one Fusion drive of the HDD’s space of 2 TB (the 2 TB of the SSD in your case is hidden!)
Incorrect. I see 4TB Fusion Drive. Also, diskutil list command shows all 3 drives (2 SSDs and Fusion Drive).
I guess I will try to split Fusion Drive into two separate disks, but this still does not explain the temps I am getting. In the previous iMac I had Apple’s NVMe SSD + Samsung 850 Evo and my temps where completely fine (and I also had Fusion Drive).
Ist dies eine gute Frage?
971 
8 Kommentare
Frankly, I don't like using M.2 SSD's in iMac's given the difficulty in getting to them. Sadly, I've seen too many failures using these M.2 adapters with M.2 SSD's. I prefer using either a real Apple SSD or OWC custom SSD as its designed straight up to match Apple's custom SSD unlike the M.2 which is not.
As to heat sinks on the SSD they are not required!
Trim should be enabled, but you are not likely facing a clean up (garbage collection) issue with such a large drive unless you've filled it quite full already.
von Dan
You didn't read my entry very well: Basically, the PCIe SSD is not seen as a discreet drive within the macOS (look at your disk information pane under 'About This Mac' Storage tab all you’ll see is the one Fusion drive of the HDD’s space of 2 TB (the 2 TB of the PCIe SSD in your case is hidden!)
I was very clear on how to view your drive config. Using diskutil will show the discreet drives but it won't show you your drives true size! You are assuming 2+2 =4TB here its 2TB only!
If you wanted a flat space you would setup a JBOD But that really only works when all of the drives are sharing the SAME I/O type which you aren't here. IF I had two or more SATA drives or two or more PCIe/NVMe drives then that would work! Here with have one SATA and one PCIe two very different I/O speeds.
Take out the SATA SSD and call it a day!
von Dan
@danj I will split the Fusion Drive and if this doesn't help, I'll post an update. Thanks for the answer
von Dominik Mańkowski
@danj ok, I have done this and the write/read speeds are fine again. Still, while benchmarking M.2 disk, the temps are going up to 79 Celsius degrees and throttling happens (while benchmarking SATA SSD, it does not even hit 50 degrees). I think I will install thermal pads and a heatsink on top of 970 Evo Plus, although I am not sure if there is a space for this. Eventually, a few thermal pads one on each other, so iMac's chassis would work as a giant heatsink - this seems like a good idea to fix the temps.
von Dominik Mańkowski
Also, one unusual thing: while I am benchmarking 970 Evo Plus, the temperature reported by 860 Evo rises from 45 to 60 degrees. Weird. The other question is how all of this will behave when I will fill 3.5TB of data again.
von Dominik Mańkowski
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